Biomimetic studies on the mechanism of stereoselective lanthionine formationElectronic supplementary information (ESI) available: separation of the diastereomers of 5; cleavage of peptides from resins; COSY NMR spectrum of the product obtained from cyclization of both E-1 and 19. See http://www.rsc.org/suppdata/ob/b3/b304945k/